1. Technical Field
This invention relates to cordless telephones and more particularly to a cordless telephone having a plurality of portable units arranged for communicating with a base unit in a frequency hopping system.
2. Description of the Prior Art
The Federal Communications Commission (FCC) recently promulgated rulings in the utilization of spread spectrum systems, including a frequency hopping system. These rulings now allow suppliers to produce improved cordless telephones which provide users much greater freedom and mobility than is available with conventional cordless telephones. Frequency hopping systems spread their energy by changing, or hopping the center frequency of the transmission many times a second in accordance with a pseudo-randomly generated list of communication channels. The result is a significantly higher signal to noise ratio than may be achieved by conventional techniques such as amplitude modulation that uses no bandwidth spreading.
Although these improved cordless telephones provide an increased operating range over what was previously available, they also provide security naturally from an eavesdropper listening in on a conversation simply because of the spread spectrum/frequency hopping transmission technique employed by these telephones. Examples of such improved cordless telephones are described in U.S. Pat. No. 5,323,447 which issued to M. E. Gillis et al. on Jun. 21, 1994, U.S. Pat. No. 5,353,341 which issued to M. E. Gillis et al. on Oct. 4, 1994 and U.S. Pat. No. 5,463,659 which issued to W. J. Nealon et al. on Oct. 31, 1995.
Communications in cordless telephones have traditionally been limited to a single one of multiple portable units in the same cordless system being able to access a common base unit in the system at any given time. With the new improved cordless telephones, more than one portable unit may attempt to access the base unit during, for example, receipt of an incoming call. If two or more portable units happen to be within the reception range of a base unit when the base unit generates a service request indicative of the receipt of the incoming call, for example, collision between these portable units in responding to the base unit may result. This is all the more likely in view of the increased operating range for these improved cordless telephones. Thus, if more than one portable unit responds to the base unit at the same time, each handset identification code generated by the portable unit and received by the base unit is very likely to be rendered unintelligible by another one or more received codes and the base unit is forced to ignore all the receive codes and go through the process of generating another service request, hopefully with more favorable results the second time. Even unrelated portable units within the reception range of the base unit may receive and initially attempt to respond to the base unit's service request. It is therefore desirable to have multiple portable units capable of operation in close reception proximity to each other and without interference in a cordless telephone system.